The progressive immunodeficiency of HIV-1 disease is associated with multiple cytopenias, in particular loss of circulating CD4+ T cells. This protocol is designed to test the hypothesis that HIV-1 infection leads to peripheral lymphopenia by inducing a central defect in T cell production. If so, thymic mass and the number of recent thymic emigrants (CD3+CD45RA+CD27+ cells) in the peripheral blood should be expected to decrease as HIV-1 disease progresses. Changes in thymic mass will be assessed by non-invasive imaging by CT scan, using well-established techniques; changes in the phenotype of circulating T cells will be assessed by multiparameter flow cytometric analysis of peripheral blood mononuclear cells. Volunteers within various age groups (20-29, 30-39, and 40-49 years old) with CD4 counts greater than or less than 500 will be chosen.